This invention relates to retorting processes for recovering product hydrocarbons from oil shale and other hydrocarbon-bearing solids. The invention most particularly relates to those oil shale retorting processes wherein coke on the retorted shale is combusted to provide heat energy.
Many methods for recovering oil from oil shale have been proposed, nearly all of which utilize some method of pyrolytic eduction commonly known as retorting. To be competitive with the production of oils from petroleum stocks, the principal difficulty to be overcome has been recovering essentially all heat value from carbonaceous material in the shale without incurring prohibitive expense or environmental damage. Since shale usually contains only about 20 to 80 gallons of oil per ton, only a limited proportion of which can be recovered as product oil or gas, economical retorting must utilize remaining heat energy contained in the shale to provide heat for pyrolitic eduction. However, sulfur emissions in flue gases released from the retorting process must be restricted to the low levels required by law while this goal is being attained.
It is known to retort oil shale by a technique of contacting up-flowing oilbearing solids with down-flowing gases in a vertical retort, and one such technique is disclosed in U.S. Pat. No. 3,361,644. To educe product vapors, the upward-moving bed of shale particles exchanges heat with a downflowing, hydrocarbonaceous and oxygen-free eduction gas of high specific heat introduced into the top of the retort at about 950.degree. to 1200.degree. F. In the upper portion of the retort, the hot eduction gas educes hydrogen and hydrocarbonaceous vapors from the shale and, in the lower portion, preheats the ascending bed of particles to retorting temperatures. As preheating continues, the eduction gas steadily drops in temperature, condensing high boiling hydrocarbonaceous vapors into a raw shale oil product while leaving a product gas of relatively high BTU content. The shale oil and product gas are then separated, and a portion of the product gas, after being heated, is recycled to the top of the retort as the eduction gas.
To minimize the volume of the recycle gas required, up-flow retorting is usually conducted with superatmospheric pressures, with the pressure in the upper regions of the retort often being between 10 and 50 p.s.i.g. However, means must be provided for introducing and recovering granular shale from the superatmospheric retorting zone without allowing valuable product and recycle gases to depressure. Conventional methods for achieving these objectives use elaborate lock vessels, valves, star feeders, or slide valves, which tend to wear rapidly and produce excessive fines through abrading the shale. Alternatively liquid sealing devices, as in U.S. Pat. No. 4,004,982 have been employed, which operate by moving shale particles through a standing head of oil or water, thereby creating a positive back pressure to forestall escape of retort gases. Liquid seals effectively contain retort gases but leave the shale wet. When incorporated into a process for combusting retorted shale in a vessel separate from the retort, as is conventional, use of liquid seals requires the expense of drying the shale prior to combustion.
To increase product yield beyond what can be educed in the retort alone, processes have been developed to generate product gases by reaction of hot, retorted shale with an oxidizing gas stream, for example, as taught in U.S. Pat. No. 4,010,092. However, such gasification reactions conducted in an oxidizing environment burn the coke on retorted shale at temperatures high enough to release significant amounts of CO.sub.2 from decomposing carbonates in the shale, thereby necessitating expensive removal of CO.sub.2 from combustible product gases.
Retorted shale contains heat value in the form of coke, and many retorting processes pass retorted shale particulates through a combustion zone to combust the coke and thus recover heat energy. However, because retorted shale generally contains sulfur components, less than complete combustion of the coke generates H.sub.2 S, which must be removed from flue gases by means of costly sulfur recovery processes. On the other hand, complete combustion may result in flue gases containing unacceptable amounts of SO.sub.2. To solve the problem of SO.sub.2 production during complete combustion, U.S. Pat. No. 4,069,132 discloses a combustion process wherein the SO.sub.2 generated during the combustion of coke on the retorted shale is converted to stable inorganic salts by reaction with alkaline ingredients of the shale. This process utilizes a combustor through which hot retorted shale gravitates cocurrently with air for combustion diluted by sufficient flue gas to control peak combustion temperature below 1670.degree. F. Under such conditions, the discharge of SO.sub.2 from the combustor is disclosed to be greatly minimized.
Because flue gases from combustion zones associated with shale retorts are usually at high temperature, many retorting processes recover heat therefrom. For example, as taught in U.S. Pat. No. 4,069,132, the hot flue gases may be utilized to exchange heat indirectly with boiler feedwater to generate process steam.
While the foregoing have met with some success, the need exists for further developments in shale retorting technology. In particular, a need exists for an oil shale retorting process for recovering shale particulates from a superatmospheric retort and subsequently maximizing the amount of heat energy recovered therefrom by combusting coke on the retorted shale. The difficulty of fully combusting coke deeply embedded in the relatively large-sized particles obtained from a retort similar to that disclosed in U.S. Pat. No. 4,069,132 has hampered the development of such a process. One can, of course, crush the shale in order to expose more coke for combustion, but retorting finely crushed shale generates unacceptably large pressure drops in the retort. On the other hand, crushing the shale subsequent to retorting poses twin problems of how to remove retorted shale to a crusher without losing valuable gases from the retort and how to separate a retort operating at superatmospheric pressure from a crusher operating at atmospheric pressure without losing a substantial amount of heat energy from the hot, retorted shale. Equipment such as star locks and wet seals would prove ineffective for this purpose because the former are subject to excessive mechanical wear and the latter remove heat from retorted shale by quenching. In addition, wet seals often consume large amounts of water, which may prove a decided disadvantage in the arid locations where oil shale retorting must often take place.
Accordingly, a principal object of this invention is to provide a dry sealing leg for removing hot, retorted shale particulates from a retort at a superatmospheric pressure without loss of retort gases and to deliver the retorted particulates to a location at atmospheric pressure with a relatively high amount of heat energy still contained in the particulates.
Another object of the invention is to provide an oil shale retorting process employing the foregoing dry sealing leg.
It is yet another object to provide an oil shale retorting process employing the foregoing dry sealing leg in conjunction with a fluidized bed combustor for maximizing the amount of coke combusted from the retorted shale particulates while minimizing the amount of SO.sub.x compounds discharged to the atmosphere during said combustion.
These and other objects of the invention will be apparent from the following description taken in conjunction with the Figures of the drawing.